Tissue imaging by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) is a technology that can be used to simultaneously explore and characterize the spatial distributions and relative abundances of endogenous compounds directly from the surface of a thinly-cut tissue slice. This technique can be used to produce visual images of various ionized species within tissue samples, including lipids and proteins. The locations and abundances of specific biomolecules can reflect the pathophysiology of the imaged tissue specimens; therefore, MALDI imaging has great potential for diagnostics, such as human disease biomarker discovery, particularly cancer biomarkers.
Currently, MALDI imaging has been used to detect only a small number of lipids and/or proteins in comparison to other mass spectrometric detection methods (e.g., MS/MS or LC-MS/MS). For example, only 212 lipids in rat brain, 550 lipids in porcine adrenal gland, 92 proteins in mouse lung, and 105 proteins in mouse kidney have been detected in single tissue imaging studies, whereas 119,200 lipid compounds have already been entered into the LipidBlast library using MS/MS, and 2800 proteins can be detected in human colon adenoma tissue using LC-MS/MS. Methods to improve the number of compounds detected using MALDI MS have focused on either manipulating the matrix used in MALDI MS, and/or using various sample preparation techniques, such as matrix sublimation, matrix vapor deposition/recrystallization, matrix pre-coating, solvent-free matrix dry-coating, matrix microspotting, automated inkjet matrix printing, and tissue pre-washing before matrix coating. Despite these prior efforts, however, a need in the art still exists for improved MALDI MS sample preparation methods and a system for preparing such samples.